Squelch discussion

Motorola Micor tone decode and squelch discussion, by Karl Shoemaker, AK2O

Introduction

"PL" is a Motorola trade mark meaning "private line" which has nothing to do with operational status of a system, being open or closed. PL does not mean "please leave". The private line name came about possibly to make commercial customers think they had a private channel and could not hear anyone else on "their" channel. In reality, F.C.C. requirement to monitor the channel before transmitter sometimes didn't work out very well. Motorola had DPL as well, for digital PL. For this discussion we will stay with TPL (tone PL). Other manufactures have the equivalent such as "Channel Guard" (GE) and "Quiet Channel" (RCA). The universal acronym is CTCSS which stands for Continuous Tone Coded Squelch System. For amateur systems that "PL" slang became another name for CTCSS.

Some explanation on how this works may be in order. For this discussion we'll call the CTCSS tone, just "tone". The user's station needs to transmit their carrier along with a continuous sub-audible sinusoidal, single tone in order to activate the system's input. The tone frequencies usually are around from 67 to 200 Hz, although recent standards have increased the range significantly. Lower frequencies are less distracting, however, higher frequencies having slightly faster access, to avoid cutting off the first word of a user's transmission. With some user discipline, any tone frequency will work. Most current radios have this feature while most older ones can be upgraded.

Commercial arrangement

OR squelch

  • For commercial use the station's squelch was set up for "OR squelch" in many cases. What this meant it still took both a carrier and tone to first open the squelch. The tone would then "disable" the squelch as long as it continued, therefore, bypassing what ever sensitivity the carrier squelch was set. This was designed for the weak mobile in and out of the station's receiver, rapidly, such as picket fencing through the mountains, or with multi-path, etc. For the commercial arrangement, using an "OR" squelch, this was to extend the system's usable range, of mobile traffic for the station dispatcher to hear, even if it's under difficult or annoying, noisy conditions. The PL reed has some "fly wheel" effect which means it will continue to vibrate and send a "valid" decode output, a few tenths of a second after the mobile stopped transmitting.

  • There was a (bad) side effect with this arrangement. Even under good signal conditions, every time the mobile would "un-key" and ending a transmission, the PL reed in the receiver would be still vibrating for another second afterwards, which would cause an annoying long burst of noise in the dispatchers ear. To null this side effect, "reverse burst" is used. Explanation is in order. When the mobile un-keys to PTT on the mic, there's a 1/4 second period where the mobile RF carrier continues to provide a signal to the base station's receiver. During this short time the mobile's PL encoder sends out a 180 phase shift on the PL tone. This slam-stops the reed immediately. After the 1/4 second the mobile stops transmitting, and causes the base receiver's squelch to close quickly. The dispatcher hears only a very short burst or a click. Really slick !

    Amateur arrangements and issues

    AND squelch

    This document is based on R and D, construction, installation and operation of the "Omak Rx" package in 1999. Remote receivers for SRG are set up for tone operation, except for the "Spokane" receivers. One set of issues appeared with using "PL", or CTCSS (tone) receivers. When the stock micor PL deck is used the only modification on the desk is removing a couple pins to disable the high-pass audio filter. On the micor audio board there are several modifications. One major reason is the type of squelch used. The PL deck's output, called PL indicate (PLI) is an active going high. This goes into IC/U 202 logic telling the squelch to stay open during a valid tone decode, regardless of the squelch (knob) setting. Thus, the name "OR" came from squelch open (with PL) or just PL present.

    That means a long burst will be heard for non-reverse burst users, because the PL reed will vibrate a little while after the user signal is gone. We don't want this. To correct this problem the audio board is changed to an "AND" squelch, meaning, both the tone and squelch needs to be active for an open audio path. Another way to see this is an "AND" is a variable sensitivity squelch. (vary with the knob on the panel). There were a few reasons for this at the time of early design. Some of them are:

  • Amateur mobiles do not use reverse burst. And even if they started it would be years before all did, causing some incompatibility for clean operation. Therefore, the receiver is set up for "AND" squelch which requires carrier squelch and valid PL tone decode, and if the carrier drops out first, that will close the squelch. This corrects the long burst problem with Amateurs using an OR squelch. Therefore, when the user's signal is gone the (carrier) squelch sets the sensitivity and point where it closes, thus creating a "normal" system squelch, with the exception of the long burst feature of that squelch IC during weak signal conditions. To change to "AND" remove the (stock) jumper between P201, pin 3 (PL output) and IC202, pin 8. The PLI output can still be used to signal outside equipment in either way, such as a repeater controller.

  • Setting the squelch allows to control the system's sensitivity, however, tone squelch only activates on the one tone. This permits other signals not intended for that receiver to be "ignored". This is a good and bad thing. When properly designed into a system such as SRG's the user can select which path/calling area he wishes to get in to. The other (remote) receivers don't respond because they are on a different tone. This keeps duplicate receiver paths from canceling each other out.

    However, for co-channel usage this can be bad also. Here's two points:

  • If a repeater system is toned to filter out interference, the interference is still there. This does not make the system sensitive. When a user attempts to compete with the interference, the interference confuses the tone decoder at the receiver, so nothing works right.

  • Incompetent frequency coordinators sometimes duplicate a coordination for a repeater system too close to another. Ignorance shows by coordinating different tones for each system would be the solution. Sadly, the opposite is true. When one user is using a repeater that cannot be heard by the users of the second system, that second set of users are unaware of their repeater being "disable" by a "foreign" user tone. Even if the first repeater is heard, the second system users cannot use that first repeater, but have to be disrupted from the distance traffic. Everyone looses.

    In other words, if another repeater (system) was on the same frequency on a certain tone, and a user from the other system (and other tone) starts transmitting, it "disables" the first repeater system. Anyone attempting to get into the first system won't (from the second other user). To make matters worse the first user would be unaware this is happening, causing user confusion and frustration. Tone access has to be properly thought out before using. Cooperation with fellow Amateur's is needed.

  • The Micor receiver is known for it's short squelch burst feature, which can be used in this case.

  • Early design used a (simple and cheap to build) a time domain voter (TDV) for the "Spokane" coverage area. You could set the squelch for any amount of quieting required to get into the receiver, so you can adjust the coverage. A fixed station would not get "stuck" on a noisy receiver. In other words, adjusting the squelch determines how noisy of a signal will be let through, or to reject it for system standards, such as keeping all (Spokane area) signals 20 db S/N or better, if not, that receiver is considered unusable, which works in a TDV voting system. For a single receiver, conventional system, this was not practical, since you would greatly reduce the system's input range, therefore, you would set the squelch for the weakest, noisy signal you would allow in.

  • Even though this document is for a receiver of a single "area" coverage unit (no voting) standardization was important for SRG's design. RnD indicates that the "AND" squelch poses no problem for future system changes and improvements. In addition, the squelch sensitivity can be adjusted at any time to increase the users input range. Currently, the remote receiver's squelch's are set to break at 5 db of quieting as long as it's tone protected as an "AND" squelch nothing should get through (weak interference) without the proper tone. With this setting, typical (usable) sensitivity is -120 dbm at the antenna port.

    It should be remembered after 2005 a real (signal-to-noise) voter replaced the TDV to provide good switching for the Spokane coverage area. As time permitted the receivers squelch's were turned down to the (out lying standard) sensitivity of the 5 dbq point. The philosophy behind this to the fact it's better to get a noisy signal though the system instead of none. Obviously, the (Spokane area) voter will select the best (quietest) user signal.

    The Micor squelch-Introduction

    When a user signal stops transmitting there is a time period where a laud burst of noise is heard during squelch closure. This typically less than a second for conventional receivers in the industry, however is still annoying to hear. The Motorola Micor radio receiver has a special squelch. It operates in two time constant modes, long and short. From threshold setting to around 20 dbq (db quieting) the circuit is in the long mode. Anything quieter than that the circuit is in the short mode. This is the best-world compromise between practical range of a system and user friendliness. The idea in the commercial world is a weak, noisy, moving mobile will still be heard in the long mode, therefore the long burst is tolerable. For strong, quiet signals the short mode provides a nice, (click) sound for squelch closure. Commercial systems normally use PL "OR" squelch in the base station to ensure the audio path is kept open during traffic. Amateur systems normally are on carrier squelch, therefore, at the mercy of the time constant of the squelch. The same goes for Amateur systems on tone, using the "AND" squelch arrangement. Others would call an AND squelch a variable sensitivity adjustment for that squelch to operate.

    A problem - in detail

    A problem was discovered around 1999 on the 147.20 repeater. While an Ellensburg mobile was talking, Spokane reception would hear occasional drop-outs. This would occur when the (Ellensburg) mobile was around 35 db of quieting while higher speed motion, say above 30 MPH, with muti-path, sometimes when a quick swish occurs. (which causes the "cos" to blank out just for a fraction of a second). For the local area coverage this was not a problem (Ellensburg to Ellensburg) however, with the additional (scanning) links the drop-out time was amplified (increased).

    At this point it's a good idea to point out the pin-out assignments for the squelch IC are difference between the mobile and base stations.

    Mobile receiver:

  • The receiver audio and squelch board contains two integrated circuits. Both are labeled with the "IC" designator. For this discussion is IC202.
  • The RUI (receiver unsquelch indicator) is on pin 10 and used for other purposes within the radio possibly for the busy light adaptor.
  • The short squelch (time constant) capacitor is designated as C229, is on pin 13 with a (stock) value of .22 uf, mylar or similar type.

    Base/repeater receiver:

  • The receiver audio and squelch board contains two I.C.s. Both are label with the U designator. For this discussion is U-202
  • The RUI (receiver unsquelch indicator) is on pin 10 and is not used.
  • The short squelch (time constant) capacitor is designated as C235, is on pin 13 with a (stock) value of .22 uf, mylar or similar type.

    Tests could not be done at the site of the affected receiver at the time of discovery and thought on the problem. Therefore a (spare) mobile receiver was used in the lab, so the "mobile" description-designator (described above) will be discussed here. When observing pin 10 of IC202 which is the "receiver un squelch indicator" (for the mobile board) this blank can be seen with an oscilloscope with a very slow DC trace. This point of IC202 in turn drives the two shunts, pins 6 and 7. Pin 7 is used for the cos output (with the 10K pull up) for SRG equipment. (It was also observed that using a much lower value for the pull up causes the speaker audio to be lower. Reason unknown at this time, but is believed to be irrelevant to the issue being discussed).

    It was concluded the best possible reason for the blanking at pin 10 was the receiver's squelch was in the short mode. To "slow" this circuit down C229 on pin 13 was increased from the .22 to 1 uf. Later at the remote site the 1 uf was just tack soldered on the PCB run, leaving the (stock) cap in place, so effectively, C235 would now be a 1.22 uf cap. (remember, it's "C235" in the compa-base version). This is the case in the "Wenatchee Rx" with the TLN6006 series audio and squelch board (non-unified chassis compa station). This caused the "short burst" to be a little longer for stability, but still pleasant to listen to. Rather than a little "click", a larger, fatter "click" would be heard on squelch closure. The "long squelch burst" is still functional, which switches in when the signal is around 20 db quieting or less. Additional research after 2009 will plan to change (not add) the value of C229/C235 to 1.00 uf using a tantalum type capacitor.

    Other squelch notes

    Analysis of the frequency response of the noise amplifiers would be in order as time permits, to see if the original blanking problem was upper harmonics fooling the squelch, from the quick multi-path fade. Incidentally, this fade is very difficult to duplicate on the bench. Having a buddy in the mobile out there helps with this test. There are scientific procedures to duplicate this in the lab, probably using a room with moving antennas either physically or electrically switching with metal reflectors to cause multi-path. Multi-path is common in the Pacific Northwest with mountains in many repeater service areas; one main reason for having a repeater in the first place !

    The condition was "duplicated" by the Author in the shop/lab. With the receiver under test, sitting on the bench a signal generator (with sufficient) level was connected to a piece of small coax, with a short (rubber duck) antenna terminated on the far end. With a medium signal quieting the receiver (not hard-limited) the antenna was swirled around quickly, in a circular motion. While observing the carrier indicator pin on an oscilloscope the DC squelch gate was observed to intermittently change state only for a brief moment (fraction of a second), therefore the swirling of the antenna simulated multi-path in the lab environment.

    During the R'nD, another interesting side effect (good or bad) was noticed. You might want to be aware of it. Originally, with a stock arrangement, with the squelch control at threshold, noisy signals have the long burst and stronger (more quieting) signals have the short one. If you turn up the squelch control (increase the noise gain to the noise amplifiers) this raises the noise reference so even more quieter signals (than earlier) still have a long or longer bust than before. Example, if the squelch control is at maximum a signal around 25 db quieting still has a long burst. This makes sense since you are increasing the noise to the noise amplifiers, which "looks" like a noisier signal to them. Now, with the modification of changing C229/C235 almost the reverse happens, when the squelch is at max the burst tends to be on the shorter side. The very short (drop out issue) should not happen as mentioned before, because of increased constant. With the squelch back to threshold it tends to be longer burst. Not a problem, just observations with the particular receiver under test.

    For the squelch IC, CAI on Pin 13 can drive a high impedance input or anything higher that 100K. If lower, it will load and affect the IC's squelch operation, disabling the short burst and making all squelch closes a very long burst, which is undesirable. If desired, this pin 13 can drive a high impedance buffer, and used to drive a cor/audio board. Since this is an analog DC output you would need to set a second "DC squelch" adjustment for desired signals. The bias, or reference on the op-amp would work nicely for this method. This method was investigated and decided not to be used for this receiver package. The shunt (pin 7) with resistor pull up was used (in the case of the mobile board).


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